I tag all Facebook advertising as facebook / display with utm tags, which enables me to see which campaigns are driving traffic and conversions from those ads.

However, I also see a large number of pageviews with facebook / referrer as the source / medium for landing pages that I have never used for organic or unpaid activity, only in those tagged paid campaigns

So, what is happening here? Is this because of people who see the ad in their feed, share it on their own feed, and then their friends clicking on it from there? Or some other route?